Network structure using high dispersion volume holography

ABSTRACT

The invention concerns a grating structure by high dispersion volume holography and applies, for example, to pulse compression or stretching devices.  
     The grating structure according to the invention comprises a transmission volume holographic grating ( 23, 33 ) produced on a reflective support ( 24, 34 ). The said grating consists of strata inclined to the plane of the support, the angle of inclination (φ) of the strata defined with respect to the normal to the support and the pitch of the strata (Λ) being chosen such that a light beam of given mean wavelength (λ 0 ), incident on the said structure with a given angle of incidence (θ), undergoes a first passage through the grating, is reflected by the reflective support, undergoes a second passage through the grating and is only diffracted by the grating during one of the said passages.

[0001] The invention concerns a grating structure by high dispersionvolume holography. It applies in particular in pulse compression orstretching devices, e.g. for the production of ultrashort or very highenergy laser pulses. It also applies in the field of opticaltelecommunications, e.g. for wavelength division multiplexing devices orwave trap devices.

[0002] In the fields of physics and chemistry, numerous analyses andmany types of processing require the use of very short, high-energylaser pulses. For example, very powerful laser pulses are required inthe field of plasma physics. Pulses with very high peak power are alsouseful in the field of machining materials, since they result in cleanerand more accurate contours, by reducing the heating of the materialsmachined.

[0003] To date, the production of very high peak power laser pulses(Tera or Peta Watts) uses the CPA (Chirped Pulse Amplification)technique. This technique consists of:

[0004] stretching a low energy femtosecond pulse to make it nanosecond,

[0005] amplifying its energy,

[0006] compressing the high energy pulse obtained to make itfemtosecond.

[0007] We therefore obtain an ultrashort, very high energy pulse, i.e.of very high peak power.

[0008] Traditionally, the pulse stretching and compression operationsare carried out using diffraction gratings, according to an arrangementrecommended by Treacy (IEEE Journal of Quantum Electronics, vol QE-5 No.9, September, 1969 p. 454-458) of which a simplified diagram is shown inFIG. 1. The arrangement represented in FIG. 1, for example a pulsecompressor, comprises in particular two diffraction gratings 11, 12arranged parallel to each other. The first grating 11 receives at anangle of incidence θ to the normal, an incident laser pulse F_(IN)(represented with a single arrow) whose wavelength components arevariable around a mean central wavelength λ₀. The diffracted beam isdiffracted in turn by the second parallel grating 12, thereby forming abeam parallel to the incident beam. This beam is reflected by reflectingmeans 13 to the second grating 12 (beam indicated with a double arrow)and follows a return path identical to the outgoing path, forming, atthe output of the first grating 11, the output beam F_(OUT). Theproperty of diffraction gratings to diffract the components withdifferent wavelengths by a different angle depending on the wavelengthis used. Thus, FIG. 1 represents the optical path for two componentswith wavelengths λ (dotted line) and λ′ (solid line), where λ′ isgreater than λ. The optical path and therefore the travel time taken bythe component at λ′ is greater than that of the component at λ, so thatthe pulse energy is concentrated in the output beam F_(OUT) in a veryshort period for all wavelengths.

[0009] The diffraction gratings used until now are engraved gratings,i.e. gratings with lines engraved on the surface at a regular pitch.However, they do not offer complete satisfaction. A disadvantage withengraved gratings is that their throughput, approximately 90%, onlyresults in a low compression efficiency, approximately 65%. Anotherdisadvantage of these engraved gratings is that they display poorresistance to the laser flow. For example, for a pulse of wavelength1053 nm and a duration of 250 femtoseconds, the gold engraved gratingsdisplay a resistance to laser flow of less than 1 J/cm².

[0010] Consequently, users equipped with installations producingconsiderable energy are unable, in practice, to fully benefit from allthe power they would expect from an efficient compressor and/orstretcher.

[0011] The main purpose of the invention is to overcome thesedisadvantages of the prior art.

[0012] To achieve this, one objective of the invention is to supply agrating structure by volume holography with high dispersive power, suchthat, in particular, it can be introduced in a Treacy type arrangementand which in addition displays very good stretching or compressionefficiency as well as better resistance to laser flow than thediffraction gratings of the prior art.

[0013] More precisely, the invention concerns a grating structure byhigh dispersion volume holography, wherein it comprises a transmissionvolume holographic grating produced on a reflective support, the saidgrating consisting of strata inclined to the plane of the support, theangle of inclination of the strata defined with respect to the normal tothe support and the pitch of the strata being chosen such that a lightbeam of given mean wavelength, incident on the said structure with agiven angle of incidence, undergoes a first passage through the grating,is reflected by the reflective support, undergoes a second passagethrough the grating and is only diffracted by the grating during one ofthe said passages.

[0014] In addition to its applications in the field of pulse compressionand/or stretching, the structure according to the invention is suitablefor other applications due to its high dispersive power, especially inthe field of optical telecommunications. In particular, the inventionconcerns a wavelength division multiplexing (WDM) device and a wave trapdevice implementing the grating structure according to the invention.

[0015] Other advantages and features of the invention will be clearer onreading the following description, illustrated by the attached figuresrepresenting in:

[0016] FIG 1, the simplified diagram of a Treacy arrangement (alreadydiscussed);

[0017]FIG. 2, a first mode of realisation of the grating structureaccording to the invention, implementing a transmission grating withinclined strata on a dielectric or holographic mirror.

[0018]FIG.3, a second mode of realisation of the grating structureaccording to the invention, implementing a transmission grating withperpendicular strata on a holographic mirror with inclined strata;

[0019]FIG. 4, the diagram of an example of WDM type device implementingthe grating structure according to the invention.

[0020] On the figures, the identical elements are indexed using the samereferences.

[0021]FIGS. 2 and 3 represent two examples of realisation of gratingstructure by volume holography according to the invention.

[0022] The grating structure according to the invention comprises inparticular a transmission volume holographic grating produced on areflective support. The volume grating is formed from a thick layer (afew dozen microns) of a holographic material in which strata of givenpitch Λ are inscribed, inclined with respect to the plane of the layersupport at a given angle φ defined with respect to the normal to thesupport. The angle of inclination φ and the pitch of the strata Λ arechosen such that a light beam of given mean central wavelength λ₀,incident on the said structure with a given angle of incidence θ,undergoes a first passage through the grating, is reflected by thereflective support, undergoes a second passage through the grating andis only diffracted by the grating during one of the said passages. Inpractice, since the angle of incidence of the beam is given (preferablyclose to the Brewster angle to avoid reflections at the interface), thepitch of the strata is determined according to the mean centralwavelength. The angle of inclination of the strata is chosen accordingto the type of reflective support used, to introduce a dissymmetry inthe direction of beam propagation with respect to the direction of thestrata during the first and second passages, which will result indiffraction during only one of the said passages.

[0023] Due to this type of structure, the diffracted beam emerges at anangle close to the angle of incidence θ. The resulting grating structuretherefore has high dispersive power, which means that it can be used ina Treacy type arrangement as described previously. The device obtainedis then highly efficient. The efficiency of a volume grating, in fact,is above 98%, which means that a compression efficiency of approximately92% can be obtained, i.e. a gain of some 40% as compared with engravedgratings. In addition, the holographic materials display good resistanceto the laser flow, for example approximately 2 J/cm² and 4 J/cm² for apulse of wavelength 1053 nm and duration 250 femtoseconds.

[0024] In the grating structure according to the invention, instead ofhaving a reflection grating acting in direct diffraction, a transmissionvolume grating is used according to the invention which, for example,allows the incident beam to pass without diffraction to the reflectivesupport, and which then receives the reflected beam and diffracts it(with an angle of diffraction that depends on the wavelength. Directdiffraction is therefore replaced by a “transmission withoutdiffraction-reflection-transmission with diffraction”, or “transmissionwith diffraction-reflection-transmission without diffraction” sequence,the transmission without diffraction and the transmission withdiffraction being carried out by the same layer containing thetransmission volume holographic grating. Apart from providing betterresistance to the laser flow than the engraved gratings of the priorart, this structure offers the advantage for example compared withreflection holographic gratings, of being much easier to producetechnologically. To obtain the same dispersion, in fact, the applicanthas demonstrated that the pitch of the strata in a reflection gratingshould be smaller, and the angle of inclination of the strata large (afew dozen degrees), making the recording of the grating very difficultin practice, at the usual recording wavelengths in the traditionalholographic materials.

[0025] According to a first mode of realisation of the invention,illustrated in FIG. 2, the transmission volume grating is a grating withstrata inclined to the plane of the support, but not perpendicular toit, produced on a reflective support which is, for example, a dielectricmirror or a holographic mirror with non inclined strata. The applicanthas demonstrated that for a beam incident on the structure with an angleθ of approximately 60.5° to the normal and an angle of inclination ofthe strata to the normal to the support of approximately 2°, therequired result is obtained.

[0026] According to a second mode of realisation of the invention,illustrated in FIG. 3, the said transmission volume grating is firstly agrating with perpendicular strata and secondly the said reflectivesupport is a holographic mirror with inclined strata. The applicant hasdemonstrated that in this example, for a beam incident on the structurewith an angle θ of approximately 60.5°, as previously, and an angle ofinclination of the strata of the reflection grating forming thereflective support to the normal to the support of approximately 89°,the required result is obtained.

[0027] We will now give a more detailed description of the examples ofrealisation illustrated in FIGS. 2 and 3 which show the componentscomprising grating structures according to the invention, adapted topulse compression or stretching means, e.g. for a device used togenerate ultrashort, very high energy pulses using a Treacy typearrangement. The structures described below are adapted for pulsesgenerated by a Neodymium-doped YAG laser (mean central wavelength ofapproximately 1.053 μm).

[0028] The components described operate for “parallel” (or transversemagnetic, TM) polarisation, such that the electric field is in the planeof incidence defined by the normal to the component and the wave vectorof the incident wave. The angle of incidence is close to the Brewsterangle, which reduces the reflection at the interface between theexternal medium and the structure. It is determined according to thedesired application. In the example in FIGS. 2 and 3, an incident beamat an angle θ of approximately 60.5° to the normal to the support meetsthe constraints of the Treacy arrangement.

[0029] The component represented in FIG. 2 comprises a support 20 whichcould be made from glass, for example of type BK7. The support 20 iscoated with a mirror 21 forming the reflective support. The mirror 21 isfor example a multidielectric mirror. According to a mode of realisationof the invention, it may consist of a stack of pairs of SiO₂-HfO₂layers. For example, the external layer may be a layer of SiO₂.According to a variant, it may also be a reflection volume grating withnon inclined strata (holographic mirror), adapted to the wavelength ofthe incident beam.

[0030] According to this example, the mirror 21 is coated with a bufferlayer 22 resistant to the laser flow, itself covered with a thick layer(several dozen microns) of a holographic material 23, which forms thetransmission volume holographic grating. The buffer layer 22 may consistof a transparent material such as SiO₂ for example, preferably depositedusing a sol-gel process. It has advantageously a refractive indexsimilar to that of the holographic material 23 and thickness such that,for the length of the laser pulse considered, the holographic grating soformed no longer causes interference between the incident wave and thereflective wave. The holographic material is for example dichromatedgelatine, photopolymer, or material produced using a sol-gel process.Tests on the resistance to the laser flow with femtosecond pulses havein fact demonstrated good behaviour of the above-mentioned holographicmaterials (for example, 4 to 5 J/cm² for the dichromated gelatine and 2J/cm² with a photopolymer).

[0031] In the example shown in FIG. 2, the material 23 used is forexample a photopolymer material inscribed with strata of pitch Λ (a fewhundred nanometres) inclined at angle φ approximately 2° to the normalto the support. Depending on the angle of incidence α of the laser beamto these strata, the beam sees in its direction of propagation strataseparated by a pitch Λ/cosα. There is a critical angle of incidence ofthe laser beam to the strata, for which the laser beam sees strataevenly distributed at a pitch equal to the wavelength (or multiple orsub-multiple of the wavelength). For this critical angle α_(cr) thegrating is highly diffractive and reflects the beam at an angle whichdepends on the wavelength. As soon as we move away from this angle, evenby 1 or 2°, the grating is no longer diffractive and it is crossedwithout modification by the laser beam.

[0032] An anti-reflection layer 24 can be deposited at the interfacebetween the external medium and the said volume holographic grating,especially when the incident beam is in perpendicular polarisation, inorder to minimise the reflection losses.

[0033] We will now give an example of numerical values for this mode ofrealisation. The thickness of the mirror 21 is approximately equal to 10microns, the buffer layer 22 has a thickness of 25 microns, the layer ofholographic material 23 has a modulation of index 0.04 and a thicknessof approximately 30 microns. The strata are inclined at an angle φ ofapproximately 2° to the normal to the support and the pitch of thestrata is approximately 630 nm for the chosen mean wavelength (1.053μm). The indices of the anti-reflection 24, buffer 22 and mirror 21layers are approximately the same.

[0034] According to the mode of realisation illustrated in FIG. 2, theincident wave is successively subject to the following effects. It istransmitted through the interface between the air and theanti-reflection layer 24. The reflection losses are minimised due to thestate of the polarisation and the angle of incidence. It is thentransmitted through the anti-reflection layer to the interface with theholographic layer, then through the holographic layer. The angle ofincidence of the beam to the strata of the holographic layer grating issufficiently different from the critical angle α_(cr) for theholographic layer to remain transparent to the beam. The difference isdue to the angle φ (angle of inclination of the grating strata); it isapproximately 2°. The incident wave then crosses the buffer layer, isreflected on the dielectric mirror and goes back through the bufferlayer to the holographic grating of layer 23. On return, however, theangle of incidence on the strata is different. It is very close to thecritical angle, resulting in very high diffraction by the holographicgrating and modification of the output angle depending on thewavelength. The emerging beams are then transmitted through theholographic layer-air interface. The reflection losses are minimised dueto the state of the polarisation and the angles of approximately 60°.

[0035] Consequently, for an input beam incident at 60.5°, the outputbeam leaves at an incidence of 51.9° for the mean wavelength of 1053 nm,the output incidence varying with the wavelength. We can thereforeproduce, for example, a transmission grating whose characteristics aresuch that nearly 100% of the wave incident at 60.5° is transmitted bythe said grating, and nearly 100% of the wave previously transmittedthen reflected by the said reflective support is diffracted by the saidgrating to leave on average at 51.9°. Gratings with high diffractionefficiency and large size (diameter approximately 150 mm) have thereforebeen produced according to the invention on dielectric mirror.Obviously, according to the principle of inverse propagation of light,we can also work with a beam incident on the structure with an angle θof approximately 52°. It will be diffracted the first time it passesthrough the holographic layer 23 through an angle which depends on thewavelength. The beams so diffracted will be reflected by the mirror 21,then transmitted without diffraction by the holographic grating 23 toemerge from the structure with an angle of approximately 60°.

[0036] We will now describe, in relation to FIG. 3, a second mode ofrealisation of a grating structure according to the invention for aNeodymium-doped YAG laser of mean wavelength 1053 nm.

[0037] The component shown in FIG. 3 comprises a support 30 covered witha holographic mirror 31 with strata slightly inclined with respect tothe layer (φ_(m)=89° to the normal to the layer) and which forms thereflective support according to the invention. The holographic mirror inthis example also disperses the wavelengths, but not significantly dueto the slight inclination of the strata with respect to the support. Abuffer layer 32, resistant to the laser flow, with characteristicssimilar to those described in the first mode of realisation, covers theholographic mirror 31. A holographic material 33, of the same type asthat described previously, is deposited on the layer 32 so as to formthe transmission volume holographic grating. In this example however,the strata are almost perpendicular to the layer (φ=0°). The angle ofincidence is almost equal to 60.5°, and the diffracted angle is almostequal to 51.9° for the mean wavelength 1053 nm. An anti-reflection layer34 may also be provided.

[0038] In this example, the dissymmetry during the first and secondpassages, in the direction of propagation of the beam compared with thedirection of the strata on the transmission grating 33 (angle ofincidence α), is obtained due to the inclination of the strata on thereflection volume holographic grating forming the reflective support 31.The examples shown are not exhaustive and this dissymmetry, whereby thebeam is diffracted by the transmission grating only in one direction,can be obtained by other configurations using the transmission gratingand the reflective support. The advantages of the examples described inFIGS. 2 and 3 include the implementation of a thin multilayer structure,highly resistant to the flow, especially since there is no need toinsert additional lamina, for example made from glass, which might notbe able to withstand the very high energies, between the layers formingthe transmission hologram and the reflective support.

[0039] The components described in FIGS. 2 and 3 apply in particular topulse compression or stretching means and to a device which generatespulses of very high peak power including such compression and/orstretching means.

[0040] Other applications are possible for the grating structureaccording to the invention, which make use of its high dispersive powerand excellent efficiency. For example, in the field of opticaltelecommunications.

[0041]FIG. 4 is a diagrammatic representation of a traditionalarrangement in optical telecommunications for Wavelength DivisionMultiplexing (WDM). This arrangement comprises collimation optics 41 anda diffracting element 42. The collimation optics 41 forms from a lightbeam of given mean wavelength output from an optical fibre 43 a parallelbeam F_(IN) on the diffracting element 42, in order to separate the beamcomponents (or channels) into the various wavelengths. On the diagram ofFIG. 4, two components with wavelengths λ and λ′ are shown.

[0042] The invention proposes a multiplexing device which implements thegrating structure by volume holography according to the invention,forming the diffracting element 42. According to the invention, itconsists of a transmission volume grating 421 produced on a reflectivesupport 422. Due to its high dispersive power, it is capable ofresolving wavelengths separated by a few fractions of a nanometre (0.2to 0.5 nm), which could be extremely useful, especially when it isnecessary to work with numerous channels. The pitch of the strata of thetransmission volume grating 421 is determined according to the meancentral wavelength of the incident beam (for example around 1.5 μm).

[0043] The invention also proposes a wave trap device for opticaltelecommunications, in order to separate a given wavelength from apacket of wavelengths. According to the invention, it comprises agrating structure by high dispersion volume holography as previouslydescribed. The characteristics of the structure are chosen so that thestructure is adapted to the wavelength to be trapped, resulting indiffraction during only one of the passages in the transmission gratingof the component of the incident beam at the said wavelength. Thecomponents at the other wavelengths are reflected without diffraction bythe structure and therefore emerge from the structure in a directiondifferent from the component which was diffracted, thereby separatingthem.

1- Grating structure by high dispersion volume holography, wherein itcomprises a transmission volume holographic grating (23, 33) produced ona reflective support (24, 34), the said grating consisting of stratainclined to the plane of the support, the angle of inclination (φ) ofthe strata defined with respect to the normal to the support and thepitch of the strata (Λ) being chosen such that a light beam of givenmean wavelength (λ₀), incident on the said structure with a given angleof incidence (θ), undergoes a first passage through the grating, isreflected by the reflective support, undergoes a second passage throughthe grating and is only diffracted by the grating during one of the saidpassages. 2- Structure according to claim 1, wherein an anti-reflectionlayer (24, 34) is deposited at the interface between the external mediumand the said volume holographic grating. 3- Structure according to claim1 or 2, wherein the said transmission volume grating is a grating (23)with inclined strata not perpendicular to the plane of the support, onthe one hand, and wherein the said reflective support is a holographicmirror (24) with strata not inclined with respect to the plane of thesupport, on the other hand. 4- Structure according to claim 1 or 2,wherein the said transmission volume grating is a grating (23) withinclined strata not perpendicular to the plane of the support, on theone hand, and wherein the said reflective support is a dielectricmirror, on the other hand. 5- Structure according to claim 3 or 4,wherein the angle of inclination φ of the strata to the normal to thesupport is approximately 2°. 6- Structure according to claim 1 or 2,wherein the said transmission volume grating is a grating (33) withperpendicular strata, on the one hand, and wherein the said reflectivesupport (31) is a holographic mirror with strata inclined to the planeof the support, on the other hand. 7- Structure according to claim 6,wherein the angle of inclination (φ_(m)) of the strata of theholographic mirror to the normal to the support is approximately 89°. 8-Structure according to one of the previous claims, wherein the saidtransmission volume grating consists of holographic materials, with atleast one of the said materials belonging to the following group:dichromated gelatine photopolymer sol-gel material 9- Structureaccording to one of the previous claims, wherein the said reflectivesupport and the said volume holographic grating are separated by abuffer layer (22, 32). 10- Laser pulse compression or stretching means,wherein they implement at least a grating structure by high dispersionvolume holography according to one of the previous claims. 11- Laserpulse compression or stretching means according to claim 10, wherein themean wavelength of the said pulse is approximately 1053 nm, the angle ofincidence of the pulse on the said structure is approximately 60°, thereflective support consists of a dielectric mirror or a holographicmirror with non inclined strata, the pitch of the strata isapproximately 630 nm and the angle of inclination to the normal to thesupport is approximately 2°. 12- Laser pulse compression or stretchingmeans according to claim 10, wherein the mean wavelength of the saidpulse is approximately 1053 nm, the angle of incidence of the pulse onthe said structure is approximately 60°, the transmission grating hasalmost perpendicular strata with a pitch of approximately 630 nm and thereflective support consists of a holographic mirror with inclinedstrata, the angle of inclination φ_(m) to the normal to the plane of thesupport is approximately 89°. 13- Device to generate laser pulses ofvery high peak power, wherein it comprises means to compress and/orstretch the said pulses according to one of claims 10 to
 12. 14-Wavelength division multiplexing device for optical telecommunications,comprising collimation optics (41) and a diffracting element (42), thesaid optics forming from a light beam of given wavelength output from anoptical fibre (43) a beam (F_(IN)) roughly parallel to the saiddiffracting element, wherein the said diffracting element consists ofthe grating structure by high dispersion volume holography according toone of claims 1 to 9, adapted to the said wavelength. 15- Wave trapdevice for optical telecommunications, wherein it comprises a gratingstructure by high dispersion volume holography according to one ofclaims 1 to 9, the characteristics of the said structure being chosen sothat the component at the wavelength to be trapped is diffracted duringone of the passages in the transmission grating, the components at theother wavelengths are reflected without diffraction by the saidstructure.